1. Field of the Invention
The present invention relates to wireless transmission in a wireless network, and in particular, to content protection for wirelessly transmitted video data based on proximity control.
2. Description of the Related Art
For copyrighted content transmission via wireless media, the content provider is typically subject to proximity control requirements so that the wirelessly transmitted content, e.g., video or music, is used within a limited range of area. An essential element to any proximity control scheme is the measurement of distance between two devices, e.g., a Source and a Destination. A Round Trip Time (RTT) test is one method used to measure the Source-to-Destination distance. Round trip time is the duration measured from when a message is transmitted by a Source until when a response from a Destination is received by the Source. Only when the RTT test is passed, authorization keys are exchanged and the content is transmitted between the devices. For example, in the Digital Transmission Content Protection (DTCP) protocol, which is widely used for point-to-point communication (e.g., between a DVD player and a digital television), the RTT needs to be less than 7 ms.
Certain existing proximity control protocols, such as the DTCP, are designed for point-to-point wired communication. In a point-to-point wired communication, the two communicating devices are largely free from interferences from other devices. However, for wireless communication networks, e.g., a WLAN, where multiple users are accessing the same medium, interferences from other devices in the network are commonplace; and, consequently, the RTT test between Source and Destination may fail because of random access delays caused by the interferences from other devices. The resulting retransmissions introduce delays, and in some instances, devices within an allowed proximity can be falsely refused. Accordingly, proximity control schemes that are designed for use in wireless communication networks are desirable.